This study concerns interactions between cardiovascular and pulmonary functions. Specific questions include the following: 1) does congestion of the lung circulation or distention of individual cardiac chambers cause reflex bronchoconstriction or decreased dynamic lung compliance; 2) does pressure within the pulmonary circulation determine the effect of lung inflation on control of breathing; 3) does positive pressure breathing influence cardiac performance by directly altering left cardiac chamber dynamics? These studies will be done in an open-chest externally perfused and oxygenated dog preparation in which mechanical and reflex interactions between heart, lungs and chest wall are minimized and in which blood gas composition is independent of pulmonary performance. The bronchomotor effects of congestion will be observed as breath-to-breath changes brought about by distention of the selected chambers in the "dry heart and lungs with blood, balloons or gas. The effects of congestion on breathing will be assessed by tying all pulmonary veins, pressurizing the vascular bed in steps and plotting the graded effects of graded inflation at each step on variables derived from the integrated diaphragmatic electromyogram. The effect of inflation on cardiac dynamics will be done by isolating the left heart with the pericardium intact and monitoring aortic output when left ventricular afterload and mean left atrial pressure are controlled while lungs are inflated in the reclosed thorax in what might best be visualized as an "in-situ Langendorf preparation".